The red one is the HORDE version. For every set there is the red one is HORDE!
Visa versa the blue one is the ALLIANCE version. These sets are earned with arena/RBGs. So in short qonquest points and after 27k earned those are earned with honor.
The green one ( 3th in line ) is the ELITE version. ( Hunter wise ) These can be bought with 27k earned.

Every set is the same , red for HORDE, blue for ALLIANCE , mostly the 3th color or the green version is ELITE!

The malevont arena/rbg gear we got now will be bought with honor aswell. So if you got an alt you want to gear, wait for 5.2. If you want to get your off set wait for 5.2